An event which indicates that the mouse wheel was rotated in a component.

A wheel mouse is a mouse which has a wheel in place of the middle button.
This wheel can be rotated towards or away from the user. Mouse wheels are
most often used for scrolling, though other uses are possible.

A MouseWheelEvent object is passed to every MouseWheelListener
object which registered to receive the "interesting" mouse events using the
component's addMouseWheelListener method. Each such listener
object gets a MouseEvent containing the mouse event.

Due to the mouse wheel's special relationship to scrolling Components,
MouseWheelEvents are delivered somewhat differently than other MouseEvents.
This is because while other MouseEvents usually affect a change on
the Component directly under the mouse
cursor (for instance, when clicking a button), MouseWheelEvents often have
an effect away from the mouse cursor (moving the wheel while
over a Component inside a ScrollPane should scroll one of the
Scrollbars on the ScrollPane).

MouseWheelEvents start delivery from the Component underneath the
mouse cursor. If MouseWheelEvents are not enabled on the
Component, the event is delivered to the first ancestor
Container with MouseWheelEvents enabled. This will usually be
a ScrollPane with wheel scrolling enabled. The source
Component and x,y coordinates will be relative to the event's
final destination (the ScrollPane). This allows a complex
GUI to be installed without modification into a ScrollPane, and
for all MouseWheelEvents to be delivered to the ScrollPane for
scrolling.

Some AWT Components are implemented using native widgets which
display their own scrollbars and handle their own scrolling.
The particular Components for which this is true will vary from
platform to platform. When the mouse wheel is
moved over one of these Components, the event is delivered straight to
the native widget, and not propagated to ancestors.

Platforms offer customization of the amount of scrolling that
should take place when the mouse wheel is moved. The two most
common settings are to scroll a certain number of "units"
(commonly lines of text in a text-based component) or an entire "block"
(similar to page-up/page-down). The MouseWheelEvent offers
methods for conforming to the underlying platform settings. These
platform settings can be changed at any time by the user. MouseWheelEvents
reflect the most recent settings.

The MouseWheelEvent class includes methods for
getting the number of "clicks" by which the mouse wheel is rotated.
The getWheelRotation() method returns the integer number
of "clicks" corresponding to the number of notches by which the wheel was
rotated. In addition to this method, the MouseWheelEvent
class provides the getPreciseWheelRotation() method which returns
a double number of "clicks" in case a partial rotation occurred.
The getPreciseWheelRotation() method is useful if a mouse supports
a high-resolution wheel, such as a freely rotating wheel with no
notches. Applications can benefit by using this method to process
mouse wheel events more precisely, and thus, making visual perception
smoother.

getWheelRotation

public int getWheelRotation()

Returns the number of "clicks" the mouse wheel was rotated, as an integer.
A partial rotation may occur if the mouse supports a high-resolution wheel.
In this case, the method returns zero until a full "click" has been accumulated.

Returns:

negative values if the mouse wheel was rotated up/away from
the user, and positive values if the mouse wheel was rotated down/
towards the user

getPreciseWheelRotation

public double getPreciseWheelRotation()

Returns the number of "clicks" the mouse wheel was rotated, as a double.
A partial rotation may occur if the mouse supports a high-resolution wheel.
In this case, the return value will include a fractional "click".

Returns:

negative values if the mouse wheel was rotated up or away from
the user, and positive values if the mouse wheel was rotated down or
towards the user

getUnitsToScroll

public int getUnitsToScroll()

This is a convenience method to aid in the implementation of
the common-case MouseWheelListener - to scroll a ScrollPane or
JScrollPane by an amount which conforms to the platform settings.
(Note, however, that ScrollPane and
JScrollPane already have this functionality built in.)

This method returns the number of units to scroll when scroll type is
MouseWheelEvent.WHEEL_UNIT_SCROLL, and should only be called if
getScrollType returns MouseWheelEvent.WHEEL_UNIT_SCROLL.

Direction of scroll, amount of wheel movement,
and platform settings for wheel scrolling are all accounted for.
This method does not and cannot take into account value of the
Adjustable/Scrollable unit increment, as this will vary among
scrolling components.